A general approach to the catalyzed production of biofuels from the ABE fermentation mixture. Source: Anbarasan et al. Click to enlarge.

Researchers at UC Berkeley have devised a new process that integrates chemical catalysis with extractive fermentation selectively to produce gasoline, jet and diesel blend stocks from lignocellulosic and cane sugars at yields near their theoretical maxima.

The process efficiently converts acetone–n-butanol–ethanol (ABE) fermentation products produced by Clostridium acetobutylicum into ketones via a palladium-catalyzed alkylation. These ketones can be deoxygenated to paraffins; these paraffins—from pentane to undecane—are components of gasoline, diesel and jet fuel. Tuning of the reaction conditions permits the production of either gasoline or jet and diesel precursors.

A paper on the work, which was supported by the Energy Biosciences Institute—a collaboration between UC Berkeley, Lawrence Berkeley National Laboratory and the University of Illinois at Urbana Champaign, and funded by BP—is published in the journal Nature.

Advances in metabolic engineering have enabled the biological production of several higher-molecular-mass jet and diesel fuel compounds from carbohydrates, but until now these processes have suffered from low titres and yields. Here we propose a chemical route to convert fermentation products from a variety of renewable carbohydrate sources into hydrocarbons that can be used for petrol, jet fuel and diesel.

Because solventogenic fermentation products have lower carbon numbers than are appropriate for these fuels, coupling chemistry can be used to produce molecules that are larger than these natural fermentation products, ideally achieved by exploiting the functionalities inherent in the starting materials. The acetone, n-butanol and ethanol (ABE) mixture produced by Clostridium acetobutylicum in a 2.3:3.7:1 molar ratio (3:6:1 mass ratio) provides such a system. C. acetobutylicum is able to produce ABE from a variety of sugars and carboxylic acids, providing the flexibility needed to accommodate regionally specific feedstocks. The ABE products harbour both the nucleophilic α-carbons of the acetone and the electrophilic α-carbon of the alcohols. These paired functionalities enable us to construct higher alkanes from two-carbon, three-carbon and four-carbon precursors by the alkylation of acetone with the electrophilic alcohols.

—Anbarasan et al.

Harvey Blanch and Douglas Clark, UC Berkeley professors of chemical and biomolecular engineering, developed a way of extracting acetone and butanol from the ABE fermentation mixture while leaving most of the ethanol behind, while Dean Toste, UC Berkeley professor of chemistry, developed a catalyst that converted this into a mix of long-chain hydrocarbons.

The process is versatile enough to use a broad range of renewable starting materials, from corn sugar (glucose) and cane sugar (sucrose) to starch, and would work with non-food feedstocks such as grass, trees or field waste in cellulosic processes.

The ABE fermentation process was discovered by the first president of Israel, chemist Chaim Weizmann, around the start of World War I in 1914, and allowed Britain to produce acetone, which was needed to manufacture cordite, used at that time as a military propellant to replace gunpowder. The increased availability and decreased cost of petroleum soon made the process economically uncompetitive, though it was used again as a starting material for synthetic rubber during World War II. The last US factory using the process to produce acetone and butanol closed in 1965.

Nevertheless, Blanch said, the process by which the Clostridium bacteria convert sugar or starch to these three chemicals is very efficient. This led him and his laboratory to investigate ways of separating the fermentation products that would use less energy than the common method of distillation.

They discovered that several organic solvents, in particular glyceryl tributyrate (tributyrin), could extract the acetone and butanol from the fermentation broth while not extracting much ethanol. Tributyrin is not toxic to the bacterium and, like oil and water, doesn’t mix with the broth.

Brought together by the EBI, Blanch and Clark found that Toste had discovered a catalytic process that preferred exactly that proportion of acetone, butanol and ethanol to produce a range of hydrocarbons, primarily ketones.

The extractive fermentation process uses less than 10 percent of the energy of a conventional distillation to get the butanol and acetone out—that is the big energy savings. And the products go straight into the chemistry in the right ratios, it turns out.

—Harvey Blanch

The catalysts work by binding ethanol and butanol and converting them to aldehydes, which react with acetone to add more carbon atoms, producing longer hydrocarbons. The current catalytic process uses palladium and potassium phosphate, but further research is identifying other catalysts that are as effective, but cheaper and longer-lasting, Toste said.

Using this controlled alkylation of acetone, n-butanol and ethanol, we developed a high-yield method for transforming readily accessible fermentation products from a variety of carbohydrates into precursors for petrol, diesel and jet fuels. By catalytically upgrading low-carbon- number fermentation products we are able to exploit highly efficient metabolic pathways and achieve near theoretical yields. Combined with the near theoretical yields attained during the alkylation reaction these higher-molecular-mass fuel precursors can be produced at relatively high titre. The tunability of this reaction to produce predominantly petrol or jet and diesel blend stocks is a significant advantage over other methods, and aligns well with current refining processes.

Although further improvements will be required for commercial implementation, the results demonstrate that in situ extraction of the products from the ABE fermentation coupled with catalytic conversion of these products can provide hydrocarbon fuel blend stocks at high yields from biomass. The integration of extractive fermentation with chemical catalysis is thus a novel and potentially enabling route for the economical conversion of biomass into liquid transportation fuels.